Story highlights

President awards Congressional Gold Medals to each of four girls, ages 11 and 14

"We just want to express incredible thanks," Obama tells families

Three former Ku Klux Klan members were convicted of murder in the bombings

President Barack Obama on Friday awarded posthumous Congressional Gold Medals to four African-American girls killed in a Birmingham, Alabama, church bombing that became a key and tragic moment of the civil rights movement.

Addie Mae Collins, Carole Robertson and Cynthia Wesley, all 14, and Denise McNair, 11, were killed during Sunday services on September 15, 1963, at the 16th Street Baptist Church.

At least 14 others were injured.

Three former Ku Klux Klan members were convicted of murder.

Obama signed legislation at a White House ceremony granting the posthumous honor and acknowledged the families who pressed for the recognition.

"To the families who are here today, those who lost daughters and sisters, we just want to express incredible thanks not only for the strength you showed in suffering but also in your persistence," Obama said.

President John F. Kennedy hoped in the aftermath of the bombing that it would spur action against racial hatred.

"If these cruel and tragic events can only awaken that city and state -- if they can only awaken this entire nation to a realization of the folly of racial injustice and hatred and violence -- then it is not too late for all concerned to unite in steps toward peaceful progress before more lives are lost," he said.